Use os_type instead of vm_type. Add os_version

Use os_type instead of vm_type. Add os_version

Use os_type instead of vm_type for more generic naming
for when we start re-imaging bare metal. Also added a
os_version dictionary for default versions of distros
that we want over-riding what downburst defaults are.

Signed-off-by: Sandon Van Ness <sandon@inktank.com>